Continued…… :

Continued…… Total internal reflection
the propagation of light along fiber is in
Zigzag fashion provided,it can under go
TIR(total internal reflection) at the
Core cladding boundry,which will
Occur only when two conditions are
Satisfied.
Condition 1: refractive index of glass fiber
Must be slightly grater than cladding refractive index(n1>n2)
Condition 2: angle of incidence(Q1)of light ray must be grater than the critical angle(Q2)

Continued…… :

Continued…… CONE OF ACCEPTANCE
Rotating the acceptance angle(Q0),
Around the fiber axis a cone shaped
Pattern is obtained ,called as
Acceptance cone ,of fiber input.
The cone of acceptance is the angle
With in which the light is accepted in
The cone and is able to travel along the fiber.the launching of light wave become easier for large acceptance cone .

Continued……. :

Continued……. Numerical Aperture(NA)
The numerical aperture (NA) of a fiber is a figure of merit ,which
represents its light gathering capability, larger the NA, grater is the amount of light accepted by the fiber.
The acceptance angle also determines how much light is able to enter the fiber and hence there is relation between the numerical aperture and cone of acceptance .
Numerical aperture(NA) = sinQ0(max)
NA =
NA=
,for

Semiconductor laser diodes :

Semiconductor laser diodes Semiconductor laser diodes-:
Are extensively used as laser source for optical fiber comm. due to following reasons-:
1)For semiconductor lasers the quantum transitions are associated with band properties of material while in conventional laser transitions occur b/w discrete energy levels
2)Compact in size
3)Divergance of semiconductor laser is considerably larger than conventional laser because of its very narrow active region.
4)Spatial and spectral characteristics of semiconductor laser are strongly influenced by properties of junction medium

Population inversion :

Population inversion To achieve the optical amplification it is necessary to create an non equilibrium distribution of atoms such that the population of upper energy level is grater than that of lower energy level ,this condition is known as population inversion
To achieve population inversion, it is necessary to exite atoms in to the upper energy level E2 and hence a non equilibrium distribution is obtained ,this process is achieved by using an external energy source and known as pumping

Optical feedback & laser oscillations :

Optical feedback & laser oscillations Laser light amplification occurs when a photon collides with an atom in excited energy state ,it causes the stimulated emission of a second photon and then both of them release two more ..phenomena being termed as Avalanche multiplication ,where electromagnetic waves (photons) are in phase and an amplified coherent emission is obtained
Optical cavity shown is
more of an oscillator than an
Amplifier as it provides +ve
feedback of photons by
Reflection of mirror hence
Optical signal is fed back
many times while receiving
Amplification is a process
through the medium.

Optical source (2) (Led) :

Optical source (2) (Led) Advantages of using LED as optical source in optical fiber are-:
1)LED’s produce more light per watt.
2)LED’s emit light of intended color without use of color filters
3)Used in applications where diming is required ,as they do not change there color when when current passing through them is lowered .
4)Being a solid state component ,difficult to damage by external shock, better response time and can be easily populated over a printed circuit board .
Disadvantages being-:
1)Currently are more expensive
2)Their performance largely depend on ambient temperature and operating conditions
3)They must be supplied with correct current
4)They cant be used in applications that need a sharp directive and collimated beam of light

Led structures :

Led structures There are no. of LED structures but only two are widely used in optical fiber communication ,they are -:
1)Surface Emitter Led’s
Used to obtain high radiance for which emission is to be restricted to small region(active region),with in the device.
The structure has low thermal impedance in active region allowing high current density and giving high emission into optical fiber

Continued…… :

Continued…… 2)EDGE emitting LED’S
Device has
Structure similar to
Conventional
Contact stripe
Injection laser
Its stripe geome-
try allows very high
Carrier injection
For given drive
Current.
It has transparent guiding layers with very thin active layer , to reduce self absorption ,which also narrows the beam divergence .

Comparison of edge emitter over surface emitter led’s :

Comparison of edge emitter over surface emitter led’s ADVANTAGES OF EDGE EMITTER OVER SURFACE EMITTER
Edge emitter LED’S have better modulation band width of order of hundreds of MHZ than comparable surface emitting structures with same drive level
Edge emitters have narrow line width than surface emitters
DROWBACKS OF EDGE EMITTERS OVER SURFACE EMITTERS
edge emitters have more complex structures
Edge emitters have manufacturing problems compared to surface emitters